1. Field of the Invention
The present invention relates to an endoscope system that calculates an oxygen saturation level of an observation target, and a method for operating the endoscope system.
2. Description Related to the Prior Art
In medical fields, diagnoses using an endoscope system that includes a light source device, an endoscope, and a processor device are widely performed. In particular, an endoscope system capable of obtaining an observation image, in which specific tissues and structures such as blood vessels and a glandular structure are emphasized, has been developed by not only capturing an image of an observation target but also adjusting a wavelength of illumination light for illuminating the observation target and subjecting an image signal obtained by capturing an image of the observation target to signal processing such as spectroscopic estimation processing.
Further, in recent years, there has been used an endoscope system that acquires biological function information based on an image signal obtained by capturing an image of an observation target so as to make a diagnosis on a lesion. As the biological function information, an oxygen saturation level of hemoglobin in blood is used, for example, as described in United States Patent Application Publication No. 2013/0030268 (corresponding to Japanese Patent Application Laid-Open No. 2013-22341). According to a method for obtaining the oxygen saturation level described in United States Patent Application Publication No. 2013/0030268, light having a wavelength at which an absorption coefficient of oxygenated hemoglobin is different from an absorption coefficient of reduced hemoglobin is used as measurement light for the oxygen saturation level, and a correlation between a plurality of image signals including at least an image signal obtained during illumination of the measurement light and the oxygen saturation level is used, so as to calculate the oxygen saturation level.
The correlation between a plurality of the image signals and the oxygen saturation level as described above may vary depending on various sites such as esophagus, stomach, and large intestine, a gender, an age, and the like, which are different for each patient, in some cases. As a countermeasure against this problem, according to United States Patent Application Publication No. 2013/0030268, before observing a body cavity with use of the oxygen saturation level, preliminary measurement of the oxygen saturation level is performed based on the image signal obtained by capturing an image of a normal part of the observation target, and the correlation is corrected based on the oxygen saturation level obtained by the preliminary measurement and an oxygen saturation level of the normal part as a standard level. Since the correction of the correlation as described above is performed, it becomes possible to calculate the oxygen saturation level regardless of the sites and the patients.
The calculation accuracy of the oxygen saturation level is decreased due to the difference in the sites and the patients as described above. Additionally, the calculation accuracy of the oxygen saturation level also may be decreased in the case where the observation target has yellow (or brownish yellow) pigment of bilirubin, stercobilin, or the like. Therefore, the observation target is cleaned in order to remove mucus or the like having the yellow pigment before calculation of the oxygen saturation level. However, it is difficult to completely remove the mucus or the like having the yellow pigment. Further, the mucus or the like having the yellow pigment may be newly secreted during the observation in some cases. Accordingly, in the case where the preliminary measurement as described in United States Patent Application Publication No. 2013/0030268 is performed in a state that the observation target has the yellow pigment, the correlation is corrected based on the image signal that has been influenced by the yellow pigment or the like, and therefore it is difficult to calculate the oxygen saturation level with a high degree of accuracy.